It is common practice to manufacture components of mechanical systems from plastic materials. Such components, although not as strong as metallic ones, still offer acceptable performances and quite often are much cheaper to produce. This economic advantage, along with the ease of production by automated techniques, renders plastic parts quite attractive in many areas where formerly only metallic elements were considered.
Many types of plastics have been used for the manufacture of mechanical devices, either thermoplastic such as polypropylene or thermosetting such as phenolics.
In the course of the design of a component made out of plastic, in certain cases it is very important to be able to adjust the density of the material to a predetermined value in such a way that the center of gravity of the end product will be at the desired position. When such an adjustment can be made, it is to be expected that the dynamic properties of the components (i.e. its behaviour when spinning or rotating) will be as required. If the density and weight of the component are not appropriate, in many instances either the whole system will have to be redesigned or the plastic part will perform very badly.
The current plastic resins have densities after curing from 0.8 to about 1.5, depending on the nature of the resins and the inert filler used. There have been reports of additions of high density materials to polymeric material to obtain plastics of higher densities but not disclosure has been found proposing a quantitative system for plastic compounding with inert fillers that would give an end product of preselected density. Rather, the presentations of the patent literature relates to increased densities by addition of high density material to plastic formulations, without precise relation to resulting gain in density versus percentage addition of high density filler. The presentation is either qualitative in terms of end densities or quantitative at best only for a specific mixture cited as an example. Nowhere is there disclosed a method of obtaining a plastic of preselected density from characteristics of a specific resin and filler.
This point is well shown by several references. In the "Handbook of fillers and reinforcements for plastics" edited by H. S. Katz and J. V. Milewski (Van Nostrand, 1978), it is said (page 193, Table 11-1) that the effect of addition of metallic particles to a resin matrix is a "function of volume loading" and that "densities increase in most cases". Other comments of the same authors on the same page indicate that the expected increase in density may vary substantially due to entrap air, moisture, etc. Therefore, it is obvious from these authors that the increase in density of a moulded plastic by addition of a high density filler will not be a simple linear relation projection but will have to take several factors into consideration, the density of the filler being only one of them. These authors present no mathematical relations relating those factors.
In a less recent text book by J. Delmonte ("Metal-filled plastics", Reinhold, 1961) it is indicated (page 4) that the incorporation of metallic particles in plastic increases the density. On page 22, this author indicates ranges of densities that can be obtained with the different metals but does not relate density to any specific formulation. Therefore, taking the teaching of Katz that density increases can be influenced by several factors and Delmonte's density ranges, there would be a rather elaborate series of trial and error tests before reaching a specific formulation for a product of specific density.
In U.S. Pat. No. 3,691,130 (D. Danilovich Logvinenko), it is teached that high density materials such as iron, cobalt, etc, can be incorporated in plastic matrix such as polyamids, epoxy resins or phenolformaldehyde resin, but no reference is made to the density of the resulting products.
A similar situation is noted in U.S. Pat. No. 3,451,934 (H. C. Hubbard), describing the manufacture of magnetic moulding material. Although the adjunction of high density iron is recommended, the effect of the iron on the density of the phenolic based material is not reported. With U.S. Pat. No. 2,910,449 (J. B. Evans) related to the manufacture of brake lining from phenolic loaded with several products, including iron, the effect of incorporation of high density filler on density of the end product is not shown. The patent granted to R. P. Lutz (U.S. Pat. No. 2,367,296) concerning leaded phenolic compounds also makes no reference to the density of the end product, and the situation is similar with C. P. Teeple (U.S. Pat. No. 2,326,000) using copper, lead or antimony as high density filler or Y. Ueda (U.S. Pat. No. 3,269,976) calling upon aluminum and copper as high density fillers.
Therefore, it would be high desirable to have a procedure enabling the formulation of moulded resinous products having a predetermined density. It would be an advantage if such a method would take into account both the densities of the resin and fillers and the variations in density induced by the moulding process, particularly the weight loss incurred by the selected resin in the course of the curing step and the porosity of the end product, whereby the trial and error approach that is used presently would be greatly simplified.